Hydrostatic Pressure and Dewatering Systems for Fiberglass Pools

We are frequently asked questions about the effects of ground water on fiberglass pools. The short answer is that it has no effect in most cases as long as the pool remains full of water.

But you may be wondering, “What if the pool is not full of water?”

My response is-- Exactly!

The ground water around a fiberglass pool needs to be removed before the pool shell can be emptied or the water level significantly lowered. This is not only true with fiberglass pools, but with any vessel…septic tanks, oil drums, gunite pools, and even vinyl liner pools. Hydrostatic pressure is the term used to describe the force that water exerts on a structure. Too much hydrostatic pressure = bad news, and like I said earlier this is not a concern with a fiberglass pool unless it is emptied. The good news is that many fiberglass pools may not require draining for 30 or 40 years down the road. That’s great, but when the time comes there needs to be a way to get rid of the ground water….which brings us to the subject of dewatering systems.

After the excavation for the pool is complete and the pool is set and level we drop one end of an 8” PVC pipe to the bottom of the excavation just outside of the deep end of the pool. The pipe stands up in the hole so the top of it extends above the top of the pool shell. We cut the top of the pipe off flush with the patio and place a skimmer lid on top of it and.... Viola...Instant access to ground water! If the pool needs to be drained simply remove the lid and drop a pump down in that baby a few days beforehand. There’s no need for a permanent pump because the ground water doesn’t hurt anything when the pool is full; plus any pump you put down there will probably go bad before you have a chance to use it anyway.

(The image on the left shows the drainage pipe just after the installation of the pool. The image below/right shows the skimmer lid that covers the pipe.)

Other dewatering systems:

There are other systems out there that work great. We prefer this method because as I stated earlier it’s cheap and easy, and because it works on the principle of pushing water rather than pulling it. Other systems utilize suction pumps like a “mud hog” that sit at ground level and draw water up from the lower lying excavation. This works well, but it is much more effective to push water up from the bottom than pull it from the top. These surface pumps also have to be much more substantial in size, they have to be primed, and some are gas powered which means they have to be refilled if pumping for days is required. The system we utilize uses a standard sump pump that connects to a garden hose which is much smaller and more cost effective means to move water. We also like this method because it pairs well with the use of clean gravel backfill. The gravel acts as a sieve and allows the water to flow unrestricted up through the bottom of the pipe. The level of water inside the pipe is naturally the same as the level outside of it. When using sand backfill it is recommended to put a layer of clean crushed gravel down for the bottom of the pipe to rest on and to drill holes in the side walls of the pipe and cover with a filter fabric to keep sand from washing in.

The bottom line is that it doesn’t matter what kind of dewatering system is installed around a fiberglass pool as long as it’s installed. As I stated earlier, ground water around a fiberglass pool is perfectly fine in most cases, but if you’re having a fiberglass pool installed at your home do yourself a huge favor and have your contractor do something to insure that the ground water around you pool can be removed if necessary.

Pool guys: What other systems have you seen that work well? I’d love to hear what you’re doing.